Laser Spectroscopy of Short-Lived Radium-224 Ions

Ra-224 is an intriguing candidate for a simple and robust optical clock with a low charge to mass ratio and transitions that are far from the blue compared to most other ions. We measured the energies of the four lowest excited states in trapped and laser-cooled Radium-224 ions (3.6 day half-life), the ²P_1/2 ,²P_3/2 ,²D_3/2 , and ²D_5/2 states. To do this we performed spectroscopy on the the ²S_1/2 → ²P_1/2 cooling transition (468 nm), the ²S_1/2 →²D_5/2 clock transition (728 nm), the ²D_5/2 →²P_3/2 cleanout transition (802 nm) as well as the ²D_3/2 →²P_3/2 transition (708 nm). We determined the transition frequencies by scanning over nearby molecular reference lines in tellurium or iodine for each transition. The ions are produced by a custom-built molecular beam epitaxy (MBE) oven which is loaded with thorium-228. Thorium-228 has a half-life of 1.9 years, and its daughter isotope Ra-224 has a vapor pressure over one trillion times greater than thorium. The newly decayed radium atoms can be heated out of the oven and the thorium is left behind. Combining this source with photoionization, we are able to realize trapped radium ions within minutes. The system has been sealed at low pressure for over a year without any noticeable reduction in ion loading rate.